Extruder screws employed in the melting, mixing, and compounding of polymeric resinous material typically employ three zones, namely a feed zone, a metering zone, and a melting zone located between the feed and metering zones. Typically the extruder screw is positioned for rotation in an extruder barrel that includes a hopper section adjacent to the feed section of the screw, and a discharge end opposite the hopper section and proximate to the metering section of the screw. During operation, solid resinous material is introduced through the hopper section and presented to the feed zone of the screw where it begins to melt. The solid resinous material is then conveyed to the melting zone where it melts at a greater rate than in the feed zone and is ultimately completely converted to a molten state. From the melting zone, the molten material is transferred to the metering zone for conveyance to a discharge end of the extruder where the material typically passes through a die.
Historically, conventional extruder screws comprised a single helical flight disposed about and cooperating with a root or body section of the screw to form a channel along which the resinous material introduced into the extruder was conveyed. As the material entered the melting section it began to melt due to the heat created by friction within the material itself, and heat from an external source conducted through the barrel. The molten material forms a melt film that adheres to an inner surface of the extruder barrel. When the film thickness exceeds the clearance between the extruder barrel and the flight, the leading edge of the flight scrapes the melt film off the inner surface of the barrel causing the molten material to form a pool along an advancing edge of the flight. As the material continues to melt, the solid mass normally referred to as the solids bed in the channel breaks into agglomerations of solid material which then intermix with the pool of molten material.
When this occurs, the amount of solid material exposed to the heated barrel is severely diminished since the solid material is in the form of agglomerations entrained in the pool of molten material. Therefore, in order to melt the entrained solid material, sufficient heat must transfer through the molten pool to the solids. Since most polymers have good insulating properties, the melting efficiency of the extruder declines once the solids bed has broken up.
In an effort to improve melting efficiency, extruder screws were developed that incorporated a second flight in the melting section that extended about the body portion of the screw and defined a solids channel between an advancing surface of the second flight and a retreating surface of the primary flight. In addition, a melt channel for conveying molten material was also formed between a retreating surface of the second flight, and an advancing surface of the primary flight. The diameter of the root or body section of the screw progressively increased in the solids channel, thereby reducing the channel's depth along the melt section, and decreased along the melt channel, thereby increasing the melt channel's depth. During operation, the melt film formed at the interface between the solid bed and the heated barrel surface would migrate over the second flight into the melt channel thereby minimizing the break-up of the solid bed.
In screws of this type the rate at which the solid material melted was determined by the surface area of the solid bed in contact with the heated barrel wall and the thickness of the melt film formed between the barrel wall and the solid bed. An increase in the surface area of the solid material in contact with the barrel wall caused an increase in the melting rate due to improved heat transfer from the barrel to the exposed surface of the solid bed. However, an increase in the thickness of the melt film between the solids bed and the barrel, acted as a thermal insulator, thereby reducing the heat transfer from the barrel to the solid material and slowing the rate of melting. Accordingly, to transform the solid resinous material to a molter state, the melt section of these extruder screws was quite long, which in turn resulted in increased cost both to manufacture and operate an extruder utilizing such a screw.
Based on the foregoing, it is a general object of the present invention to provide an extruder screw that overcomes the problems and drawbacks of prior art screws.
It is a more specific object of the present invention to provide an extruder screw wherein the solid material introduced into the screw is melted and mixed in an efficient manner.